So-called progressive addition lenses are now common. Such lenses hide the spherical addition common to bifocals by having a progressive addition of sphere from a view through distance portion at the top of the lens to view through near viewing or reading portion at the bottom of the lens. These lenses have the aesthetic advantage of hiding the easy to observe optical line boundaries characteristic of traditional bifocals.
A moments reflection upon the normal optical construction for bifocals can help with the understanding of such lenses.
Bifocals typically have an upper segment for distance observation. Relative to a lower segment of such lenses, bifocals have a less positive or more minus power of sphere in the upper segment of such lenses.
At lower segments of the lens, bifocals have more plus or less minus power of sphere. This sphere power difference substitutes for the loss of accommodation that naturally occurs with the aging process. Thus, a person having partially lost or totally lost accommodation can remain relatively relaxed and still read with the aid of the lower spherical add segment of such bifocal lenses.
In normal bifocal lenses, the demarcation between the distant portions of the lenses are regular optical boundaries that have the easy to recognize line border on the lens. An operator of a lensmeter can register the segments utilizing the line boundaries and readily obtain optical measurements of each of the discrete segments of the lens.
The so-called progressive additional lens has the boundaries between the discrete optical zones of the lens hidden by a gradual change of the optical power. This lens does not include the telltale line boundaries between the bifocal portions. The lack of this telltale line makes the difference in the boundaries essentially invisible. One cannot immediately perceive that a person with such glasses is wearing bifocals. Further, the progressive addition lenses allow the wearer to utilize the transition power of the lenses. Thus, where intermediate accommodation is required, the wearer of such lenses can learn to direct his vision at the correct elevation relative to such lenses to achieve accommodation between two extremes.
Unfortunately, when such lenses are measured by a lensmeter--particularly an automated lensmeter--the particular portions of constant power can not be accurately located. Error in the measurement results when the lens is incorrectly placed. Further, and because there are no boundaries to guide measurement, such misplacement is a frequent occurrence.
It is known in the prior art to have a light on a lensmeter to indicate to the operator of the lensmeter when that operator is trying to measure in a so-called "non-toric zone" of a progressive addition lens. Such lights, however, do not tell the operator in which direction lens movement must occur to make an accurate measurement. The best that such lights do is to indicate when the person is in a so-called non toric zone.
The theory behind such indications of non toric zones is set forth in Humphrey U.S. Pat. No. 4,180,325 issued Dec. 25, 1979 entitled Lensmeter with Automated Readout. In this patent, Humphrey (one of the inventors herein) sets forth a mathematical approach for the analysis of toric and non toric lenses. The specific combinations of ray deflection which signal the presence of non-toric lens effects are given there in and defined as;
PV.sub.1, PV.sub.2, and C.sub.A where PV terms are proportional to components of power variations across the lens surface and C.sub.A is proportional to circular astigmatism.
In the prior art, the so-called non-toric zone presence is signalled by a light that comes on when (PV.sub.1).sup.2 +(PV.sub.2).sup.2 +(C.sub.A).sup.2 exceed a predetermined value. Details can be found in the referenced application. Directional information is omitted; required lens position must be found by the operator's movement of the lens without assistance.